DISTRIBUTION CHAGES
Variation in geographical range can cause what appears to be abrupt change. Ostracoderms were an ancient group of marine jawless is that lived in different areas of what is now Europe. In this sequence, color indicates the gradual evolutionary change in structure of these vertebrates as shown by fossils found in the Baltic area. This was near the centre of the distribution range of these ostracoderm fish, and here fossils have been found showing all stages of their evolution. (The Baltic area is represented by the central column on the diagram).
The sequence of fossils to the right of the central column shows apparent sudden changes between forms. These fossils came from what is now Scotland, an area only colonized by these creatures during occasional expansion in their range. These fossils give an erroneous impression of punctuated evolutionary change simply because of where they were found.